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Is yours manual or Automatic HVAC system ? heating ventilation air conditioning . When is the last time you tried it ? did it work then , last year ? Where do you live south , north ? Have you had any refrigerant added lately ?
The Heating, Ventilation and Air Conditioning (HVAC) system is divided into three separate sections. The first, Heating, Ventilation and Air Conditioning, has all procedures that pertain to a HVAC component or function that is not specifically associated with the manual control system. The second, HVAC Systems-Manual, has all procedures specific to the manual control system.
Begin the system diagnosis with the:
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Symptoms - HVAC Systems - Manual in HVAC Systems-Manual for 4.3 L without DTCs

The Diagnostic System Check will provide the identification of the control module(s) which command the system.
The use of the Diagnostic System Check will identify the correct procedure for diagnosing the system and where the procedure is located.
Review the Description and Operation information to help you determine the correct symptom diagnostic procedure when a malfunction exists. Reviewing the Description and Operation information will also help you determine if the condition described by the customer is normal operation.

if there is a bridge rectifier or diodes directly down line of the transformer i would suspect it is bad.. put an inline fuse also on the primary side of the transformer and wouldn't hurt to put a fuse on the secondary also if the transformer only has two wires for the secondary..

Hello. Sounds like you do have power. That spring loaded plastic switch is a 24v contactor. It pulls in when energized. If you push it in manually everything shoud run normal. Ck your 24v transformer. On one side (feed) you should have 115v or 220v depending on what kind of transformer is used likely (115v). Coming out of the transformer you should have 24v. If not you need a new transformer. You will need a mutimeter for this. If you know you have power where the wires are connected then you know how to use a mutimeter. Only way to know for sure outside of manually pushing in the contactor. Good luck. Be sure to kill the power before making repairs.

If you don't have a volt meter, purchase one. I've seen them as low as $20.00 at Sears and even lower at Home Depot and other discount stores. The HUM terminals are most likely low voltage (24v). The "N" is the neutral. Assuming you have confirmed this is low voltage (24v, you will use HUM and "N" terminals. This control sequence is typically to allow for operation of your humidifier only when there is a call for your fan to run. The brown wires coming from your Humidifier are your low voltage circuit. One should be directly wired to "N". The other should be ran to your humidistat. The other side of the switch of your humidistat should be wired to the HUM terminal.
I hope you will find this helpful. Good Luck!!!!

If you chose to install this with an auxilary transformer. Turn the main power off to the furnace. Mount the transformer on the handi box to the furnace or inside the furnace cabinet. Wire the primary wires from the transformer to the main power coming into the furnace. Connect black to black and white to white. Wire the secondary side of the transformer in "series" with a #50 relay, the humidistat, the solenoid valve and back to the transformer. Determine which wire powers the blower motor for the heating speed. Clamp the #50 relay to the wire.
Good luck
HVAC Teacher